D346 MARINE ENGINE Parallel Operation Caterpillar

Parallel Operation
1.1. Initial Start-Up
2.1. Phase Rotation
3.1. Frequency Adjustment
4.1. Voltage Adjustment
5.1. Starting Multiple Units
6.1. Paralleling
7.1. Load Division-(Speed Droop)
8.1. Circulating Currents
9.1. Parallel Operation of Governors
10.2. Summary
11.1. Electronic (2301) Governor
12.1. Stopping

Initial Start-Up

Preparing a generator for parallel operation requires special attention. Before attempting to parallel units for the first time, all units must be checked to be sure the following three conditions are met:

1. Same phase rotation
2. Same alternating current frequency.
3. Same voltage adjustment

Phase Rotation

The phase rotation must be the same. A set of three light bulbs is used to determine whether the phase rotation of the incoming unit and the phase rotation of the line are the same.

1. Connect the light bulbs between the generator leads and the corresponding line phase, i.e., terminal 1 to line 1 across the open circuit breaker.
2. Start the units to be paralleled and bring them up to speed. As they approach the same speed the lights will start to blink.

a. If the lights blink in sequence one of the units is connected backward. To correct this remove generator leads 1 and 3 at the circuit breaker and exchange them. This reverses the direction of phase rotation. Line 2 should always be connected to line 2.

------ WARNING! ------

Never attempt to work on electrically hot wiring. Stop the electric set before rewiring generator leads. Open circuit breakers before working on the equipment which they control.

b. If lights blink in unison the phase rotation of both engines is the same, and condition 1 has been met.

Frequency Adjustment

The speed of units to be paralleled must be the same. Speed refers to the alternating current frequency.

1. Allow each electric set to run under load long enough for the internal temperatures to stabilize (about one hour).
2. Adjust the governor control to give rated frequency at full load.
3. Remove the load and check the high idle speed; it should be approximately 3% above full load speed. If these speeds can not be obtained, contact your AVSpare dealer.
4. For the most consistent results repeat steps 2 and
3. Condition 2 has been met.

Voltage Adjustment

The voltage level and voltage droop adjustments determine the amount of circulating currents between generators. Carefully matched voltage regulator adjustments will reduce the circulating currents. Loads of .8 power factor (primarily motors) require a generator voltage droop of about 5%. Voltage droop is expressed as the percentage of voltage change from no load to full load. Use the same voltmeter to make adjustments on each unit to be paralleled.

1. Adjust voltage as described for Single Unit Operation, Initial Start-Up.
2. With the engine running at high idle, turn the voltage droop clockwise about 1/2 of full range.

If driven load is to be unity power factor, set the voltage droop control on all generators at 1/2 of full range and proceed to Step 7. If driven load is to be normal (0.8 power factor) proceed to Step 3.

3. Readjust the voltage level control until the voltage is about 5% above desired voltage.
4. Apply full load at .8 power factor.

If a generator is to be paralleled with other generators, the voltage droop of each generator must be the same to satisfactorily divide reactive load.

5. Readjust the voltage droop control to obtain desired voltage with full load at .8 power factor.
6. Repeat Steps 3, 4 and 5 for each generator to be paralleled until line voltage is equal to desired level at .8 power factor and no load voltage is approximately 5% above rated voltage.
7. Parallel generators and apply the driven load (see the topic Operation/Paralleling). If the sum of the amps of the individual generator amperes exceeds the total amps going to the load by 10% at full load, adjust voltage droop controls to share current proportionally between generators.


Do NOT exceed rated ampere load on any single generator.

8. Tighten the locknuts on all controls and install the access cover. Condition 3 has been met.

Starting Multiple Units

Starting-Units are started the same as single units.


Units may be paralleled at no load or paralleled with units under load. To parallel two or more units the following conditions must be met:

1. Same phase rotation.
2. Same voltage level.
3. Same voltage droop.
4. Same frequency.
5. Voltages must be in phase.

The first three conditions have been met in the initial start-up for parallel operation.

1. Start the unit to be paralleled according to the procedure in the engine operation section.
2. Turn the synchronizer lights on.
3. After the engine has run long enough to warm up, bring it up to synchronous speed (the same frequency as the unit on the line). The synchronizing lights will begin to blink.
4. Using the governor control adjust the speed until the lights blink very slowly.
5. The lights are off when the voltages of the two units are in phase. At this point very quickly close the breaker while the lights are out.
6. Use governor controls to share KW load between engines
7. After generator temperature has stabilized (1 hr.), adjust the droop control of each generator so as to share the reactive load and to limit the circulating currents. Less droop (moving control CCW) increases the reactive current carried by the generator.

The frequency of the incoming unit should be slightly greater than the line frequency. This will allow the incoming unit to assume some of the load rather than add to the system load.

Load Division-(Speed Droop)

Once two units have been paralleled their share of the load is determined by the governor control setting. If two units of the same capacity and the same governor characteristics have the same governor control settings they will share the load equally.

To transfer the load from one engine to the other follow this procedure:

The total load must not exceed the capacity of the engine.

1. Increase the governor speed control of one unit to increase the load.
2. Reduce the governor speed control of the other unit to decrease the load on that unit.
3. Raise or lower the governor speed control of both units to change system frequency.

Circulating Currents

When two units are paralleled there will be circulating currents. These currents are not doing useful work, but are flowing between the generators. By determining the total generator amperage and subtracting the amperage going to the load, the amount of circulating current can be determined.

Circulating currents are caused by voltage differences between the two units.

With cold generator sets, circulating current may be as high as 25% of rated amperes, without being considered harmful. Circulating current is part of the total generator current and this total must not exceed the amperage rating.

As the generators warm, the circulating currents will decrease. The ammeter readings should decrease slightly, but the voltage meter readings should remain constant.

Parallel Operation of Governors

This section is a general description of the function of the engine governor in relation to load division between parallel electric sets. For detailed information on governor controls and adjustments, see the Operation Guide and Service Manual for the engine.

It is very important that two basic facts be understood concerning load division between generator sets operating in parallel. First, the power supplied to the generator and thus to the load is a function of the engine. The engine governor settings and the positions of the governor controls determine the amount of power delivered by the engine and the KW load carried by the generator. If the governor control setting is advanced, the engine and generator will assume more KW load. Likewise, decreasing the governor control setting will result in a reduction of load on the unit. Any other units on the line will, conversely, either reduce or gain load at the same time, assuming no change in total load or no change in the governor settings of the other units has taken place. Second, the division of power is not determined by generator excitation or terminal voltage. The Power Factor at which a generator will operate when paralleled with other generators is determined by its excitation. For more discussion on this subject, refer to the section on Parallel Operation of generators.

Governors furnished with AVSpare Powered Electric Sets can be either of two types, governors with fixed speed droop or governors with adjustable speed droop. The values of speed droop used are commonly 3% and 0%. Governors with adjustable speed droop can be adjusted so their characteristics match quite closely the characteristics of governors with fixed speed droop. The operating characteristics of the following combinations of governors on paralleled electric sets will be described.

1. Two 3% governors.
2. One 3% governor and one 0% (isochronous) governor.

Example 1-Two 3% Governors

When paralleling A.C. Generators, the engine governors must have the same speed droop characteristics if the sets are to divide the load in proportion to their ratings throughout the entire operating range.

The governor speed droop characteristics of these two units are similar. This is shown by lines "A" and "B" coinciding in Figure 1. If both units are started, set for high idle speed and paralleled at no load, the system frequency will be 61.8 cycles or 103% of rated frequency. As load is applied to the system, the frequency will decrease along the speed droop characteristic until the frequency at full load is 60 cycles.

Figure 1-Paralleling two units with similar speed droop characteristics.

If Unit A had been operating alone carrying full load, the system frequency would be 60 cycles as shown by Line "A" at 60 cycles and 100% load. Now, if the frequency of Unit B is adjusted by the engine speed control to be equal to that of Unit A and the circuit breaker of Unit B is closed, the system would be operating under the following conditions:

1. Unit A is at 60 cycles and 100% load.
2. Unit B is at 60 cycles and 0% load. The characteristic of the governor of Unit B at this time is shown by the dotted line B.

In order for Unit B to carry load, it is necessary to advance the speed setting of the governor. If it is advanced to the full load position, the governor characteristic of B will coincide with the characteristic of A. Since the load on the system was 100% of one unit, no change was made in total load, and the available capacity is now 200%, the system will operate at 50% load on each unit, and 60.9 cycles for two units of equal capacity (Point E). For units of unequal capacity, the load will be divided in proportion to the ratio of the capacity of each to the total capacity. The system frequency will be determined by the points on the governor characteristics corresponding to these loads. The frequency will be the same for both units since paralleled alternators must operate at the same speed.

If Unit A had been a 60 KW unit fully loaded and a 100 KW Unit B was paralleled with it and the governor adjusted to the full load position, the final load division and frequency would be determined as follows:

37.% of the capacity of either unit. Again, using Figure 1 for the governor characteristic of the 100 KW unit and reading up from the value of load (37.5% to point F), we find the system frequency to be 61.125 cycles.

Figure 2-Paralleling two units, one with a 3% governor and the other with a hydraulic governor set for isochronous operation.

Example 2-One 3% Governor and one 0% (Isochronous) Governor.

The characteristics of the 3% governor (Unit A) is shown by line A of Figure 2 and the characteristics of the isochronous governor (Unit B) is shown by line B. Only at full load, 60 cycles, do the frequencies of the units have the same value. It is customary to operate a system of this type with a system load greater than the capacity of Unit A. In this way Unit A carries its full load at 60 cycles and the additional load and load swings are handled by Unit B, also at 60 cycles. The system can maintain constant frequency by this method of operation. The system described in example 1 cannot maintain constant frequency with load changes because of the speed droop characteristics of the governors.

In the system described in this example, if the load is less than the capacity of Unit A (which has the 3% governor) and can be carried by Unit B, the governor setting of Unit A can be reduced to give a governor characteristic such as A, so Unit A will still carry the steady part of the load and Unit B will carry the load swings. If the system load is reduced to the point where Unit A is not operating at the 60 cycle point of its governor characteristic, Unit A will try to motor Unit B and the system frequency may be greater than 60 cycles. The reason for using an isochronous governor in a power system is to maintain constant frequency above approximately 40% load.


The preceding discussion and examples of governor operation can be summarized as follows:

1. The simplest governor combination for paralleled electric sets is to have a 3% speed droop characteristic for each governor. If a constant frequency from no-load to full-load is required, one governor can be adjusted for isochronous operation. This is called a "lead unit".
2. In order for all paralleled units to accept their full share of the load, the following governor adjustments are required:
a. The same full load speed.
b. The same high idle (no-load) speed in the case of governors adjusted for speed droop operation.
c. Governor controls set to the high idle position so the full governor range is available.
3. Operation of an isochronous governor in parallel with speed droop governors requires the special techniques described in example 2.
4. Any number of electric sets can be operated in parallel. However, only one governor of the group can be adjusted for isochronous operation except in the special cases of electronic governors with automatic load sharing.

Electronic (2301) Governor

When using Electronic Governors (2301) load sharing will be done automatically if it is a load sharing Governor.


To remove a generator from the line do the following:

1. Check the load. It must be less than the rated capacity of the remaining units.
2. Be sure the neutral of one of the remaining units is grounded.
3. Remove the load from the outgoing unit as described in LOAD DIVISION. The amperage may never go to zero due to circulating currents.
4. Open the circuit breaker.
5. Allow the engine to cool for 5 minutes.
6. Stop the engine.